1. Technical Field
The invention relates to flat cutting elements of flat shape, suitable for use more particularly for forming the tip of twist drills, of composite nature, comprising a central abrasive blade having a content greater than 80% vol. of a ultra-hard material sandwiched between two lateral support layers.
2. Prior Art
Cutting elements of the above type are already known whose ultra-hard material is a polycrystalline diamond compact, the term "compact" designating a sintered product formed by grains bounded together by bridges, created by diffusion of material in the plastic condition. Document No. EP-A-0,180,243 describes for example a composite cutting element having a central blade consisting of a diamond compact connected to metal carbide support layers via intermediate layers formed of a material whose heat expansion coefficient provides a transition between the support and the blade and which prevents diffusion of the catalyst binder between carbide and diamond during manufacture of the element by welding. The ratio between the added thicknesses of the lateral layers and the thickness of the blade is generally between 1 and 2.
Such a cutting element is unfortunately hardly suitable for drilling materials containing iron, due to the chemical reactivity of diamond. A compact of cubic boron nitride, abbreviated to CBN, is known to have a chemical stability greater than that of diamond when in presence of iron. Consequently it has already been proposed to use CBN compacts to replace polycrystalline diamond compacts whenever a ferrous metal is to be machined. The CBN compacts are produced by adding a catalyst binder to the CBN. Among the catalyst binders, a mixture of elementary silicon and materials supplying carbon and aluminium is particularly favorable (EP-A-0,181,258).
If an attempt is made to substitute a CBN compact for a diamond compact in a flat cutting element having lateral tungsten carbide layers, problems. If the carbide layer is directly bounded to the blade, binder contained in the carbide diffuses, during sintering into the abrasive layer and CBN in the final cutting element is no longer thermostable. If, in order to prevent such diffusion, intermediate layers are provided, i.e. if a construction is adopted to the kind described in document No. EP-A-0,180,243 for diamond containing cutting elements, the thickness is increased to such an extent that the flat cutting element becomes difficult to use.
It has also been proposed to locate a CBN cutting blade on a support via a molybdenum layer (JP-a-60 263 601 and JP-A-60 263 602) under conditions of pressure, temperature and duration which appeared insufficient for providing a compact.